1. Field of the Invention
The present invention relates to an optical information medium having a recording layer made of a dyestuff or coloring matter, such as a recordable CD (compact disk), i.e., so-called a CD-R, and in particular relates to an optical information medium provided with the dye recording layer, a reflective layer and a protective layer, one by one on a transparent substrate.
2. Description of Related Art
A disk of a single-plate type in conformity with an orange book, i.e., a CD-R, has, as shown for example in FIG. 3, a transparent substrate 1 of polycarbonate in the shape of a disk, having a center hole (not shown in FIG. 3), on which is provided a dye recording layer 2 of an organic dyestuff such as a cyanine dye or the like. On the dye recording layer 2 is provided a reflective layer 3 of a metal film, and further on the reflective layer 3 is provided a protective layer 4 of a UV (ultraviolet) curable resin. On a main surface of the above transparent substrate 1, there is formed a guide groove 6 in a spiral shape as a tracking means, and the dye recording layer 2 is formed on the main surface having such the guide groove 6.
Conventionally, with an optical information medium having the dye recording layer 2, such as the CD-R mentioned above, it is common to use a film of gold as the reflective layer 3.
For the purpose of cost reduction of such an optical information medium having the dye recording layer 2, such as the CD-R mentioned above, it has been studied to apply a film of silver as the reflective layer 3 other than a gold film. However, silver is inferior in chemical stability to gold, therefore, there is a problem of corrosion due to hydrogen sulfide and so on during the long-term use thereof.
As mentioned above, the protective layer 4 of the UV curable resin, which is provided on the reflective layer 3, is effective to protect it against shock or impact and to prevent the reflective layer 3 from damage thereon. However, it is not necessarily effective in preventing corrosive molecules and/or ions of such as hydrogen sulfide and so on from penetration when used as a barrier for protecting the reflective layer 3 from the corrosion thereof. This is mainly because the UV curable resin forming the protective layer does not fully show a perfect airtight property, therefore, the corrosive molecules and/or ions penetrate therethrough, as well as, it is conceivable that the corrosive molecules and/or ions penetrate into a side of the reflective layer 3 through defects 7, such as minute concave or pin-holes which are caused when the protective layer 4 is formed. Further, forming the reflective layer 2 with silver showing a higher chemical activity, the dyestuff of the dye recording layer 2 easily reacts with the silver of the reflective layer 3, therefore, the reflective layer 2 and the reflective layer 3 easily deteriorate.